Also use DialContext instead of just Dial so that we can ensure
the call returns before we close the listener.
The Dial in this test is intended to complete before the call to
Accept, but there is no synchronization to ensure that and sometimes
it doesn't happen. That's ok and mostly immaterial to the test, but it
does mean we need to ignore Dial errors (which can happen when the
listener is closed), and we need to be a little more careful about not
dialing a port that may have already been reused by some other test.